The present invention relates to electro-magnetic motors, particularly to cooling apparatus for a linear electric motor.
Heating or cooling of air next to a linear motor coil can be caused by poor temperature stability of the linear motor coils. When a linear electric motor is associated with a stage of a lithographic system used for positioning semiconductor wafers for lithographic processing, the interferometric metrology (location measurement) of the lithographic system will be degraded in performance by air temperature fluctuations in critical areas within an environmental chamber. The flow rate of the air in the chamber is sufficiently high so that air turbulence is unavoidable. The details of the turbulence depend on the chamber and stage mechanism geometries. The turbulence itself does not affect the interferometer beams which are used to monitor the exact position of a wafer stage. However, if the air in the chamber varies in temperature, and therefore in density and refractive index, the turbulence mixes air with different properties, leading to fluctuations in the interferometer optical path length.
Heretofore various linear motor cooling systems have been proposed. These are typified by U.S. Pat. No. 4,916,340 where a combination of insulating material and a variable flow liquid cooling mechanism are utilized to eliminate the problems associated with excess heat generated by the linear motors. A "thermostat" approach is employed to maintain a zero temperature differential between a surface plate and the linear motor. The cooling medium is contained in a cooling medium tank which is maintained at a constant temperature by a separate device (not detailed); this cooling medium is circulated via flexible tubes by means of a pump and four (4) solenoid valves. These valves are in turn controlled automatically by a temperature controlling circuit. When power is applied to coils of a linear motor mounted on a stage, the yoke supporting the associated magnet track moves rectilinearly and at the same time produces heat at the coils. If this heat is transmitted to a base supporting the stage, it produces a deformation of the base which degrades the moving accuracy of the stage. As the solenoid valves are opened, the cooling medium is fed under pressure into flexible tubes and then into various linear cooling conduits whereby the heat is removed. As a result the temperature difference between each linear motor and the base is reduced to zero.
U.S. Pat. No. 4,839,545 describes a cooling system for linear motors where an armature of a linear motor is cooled by serpentine channels formed in laminations of the armature. U.S. Pat. No. 4,625,132 describes a linear motor with a seal for an opening in a U-shaped channel and for directing a controlled flow of cooling gas under a positive pressure to a wound field-generating element (coil) of the motor between the stator and moving element.
While others have constructed linear motor coils with aluminum or other metal cooling jackets, the movement of the included jackets causes electric eddy currents which introduce drag. The linear motor has to be driven harder to generate greater force to compensate for the drag and as a result uses more energy. This reduces the motor efficiency. Efficiency is maximized by placing as much as possible of the coil wire in the moving gap between the linear coil and the magnet track, and efficiency decreases approximately as a square of the gap between the magnet track and the coil.